Hi Florent,
b) We motivate users to write their own JSON serializers in XQuery...
As I failed miserably in my attempt to motivate any user out there (it could be that I was too impatient, though), I just wrote a simple XQuery function for serializing XML to JSON. The JSON output is similar to the one generated by your online tools. Please be warned: The code is experimental (it ignores the famous 10% of corner cases), so any patches are welcome.
Have fun, Christian